Ignition coils produce the spark necessary to begin the combustion process. Ignition coils are a member of both the primary and the secondary ignition circuits. Today's coils multiply 12 volts battery voltage to 25k to 40k volts through induction.
It's constructed of two coils of wire wrapped around a soft iron core. The primary winding contains fewer (a couple hundred) windings of thicker insulated wire wrapped around the secondary windings. The secondary coil contains thousands of fine windings wrapped around an iron core.
When turned on, the primary circuit creates a magnetic field. This magnetic field passes through the secondary winding and soft iron core. This induces a voltage in the secondary windings. When the primary circuit is suddenly opened, voltage passes through and is multiplied to thousands of volts by the secondary's many fine windings. It's then passed on to the spark plug through the secondary circuit. The more windings a coil has, the more voltage it produces.
The primary circuit is switched on and off by the ignition control module. The module uses signals from the crankshaft and camshaft position sensors to locate cylinder #1 during startup. The PCM often contains the ignition module. Once the engine starts, it uses these along with other sensors such as the ECT and the TPS to determine dwell (on time) and spark intensity.